Telephone system



Sept. 14, 1937. o, A PEARCE ET AL 2,093,249

TELEPHONE SYSTEM Filed Sept. 12, 1935 14 Sheets-Sheet 1 Sept. 14, 1937.

O. A. PEARCE ET AL TELEPHONE SYSTEM Filed Sept. 121955 14 Sheets-Sheet 2 fan: Team? 160mm:

Sept. 14, 1937. o. A. PEARCE E1 AL 2,093, 9

TELEPHONE SYSTEM Filed Sept. 12, 1955 14 Sheets-Sheet 3 Sept. 937. O. A. PEARCE ET AL 2,093,249

TELEPHONE SYSTEM 14 Sheets-Sheet 4 Filed Sept. 12, 1955 dbz Sept. 14, 1937. o. A. PEARCE ET AL TELEPHONE SYSTEM Filed Sept. 12, 1935 14 Sheets-Sheet 5 5 a A 2 0mm ap W 5 2 W3 [4% .MT #6 i p Sept. 14, 1937. o. A. PEARCE in AL TELEPHONE SYSTEM Filed Sept. 12, 1955 14 Sheets-Sheet 6 [pf/.5 Rafa w? Fume Sept. 14, 19.37.- o. A. PEARCE ET AL TELEPHONE SYSTEM Fi] .ed Sept. 12, 1935 14 Sheets-Sheet '7 INM Sept. 14, 1937. o; A. PEARCE El AL I 2,093,249

-TELEPHONE SYSTEM Filed Sept. 12, 1935 14 Sheets-Sheet 8 "f/VVE/V 70 95 0W6 AM: 3544 65 Sept. 14, 1937. o. A. PEARCE ET AL TELEPHONE SYSTEM Filed Sept. 12, 1935 14 Sheets-Sheet 9 Sept. 14, 1937. o. A. PEARCE ET AL TELEPHONE SYSTEM 14 Sheets-Sheet 10 Filed Sept. 12, 1935 "/NVf/VTQPS- 014 5 44/45 #54205 [05/5 77am! AJ/CH/IIGAS p 4, 1937. o. A. PEARCE ET AL 2,093,249

TELEPHONE SYSTEM Filed Sept. 12, 1935 l4'SheetsSheet 12 BA I I T SL2 5L5 5L4 8L5 SL6 51:1 513 51.9 suo 70H; Z'ewae lea/Has Sept 14, 1937. y l o. A. PEARCE ET AL 2,093,249

TELEPHONE SYSTEM Filed Sept. 12, 1935 l4 Sheets-Sheet 15 /s Term? Rwy/was Sept. 14, 1937. o. A. PEARCE ET AL TELEPHONE SYSTEM 14 Sheets-Sheet 14 Filed Sept. 12, 1935 INS Patented Sept. 14, 1937 TELEPHONE SYSTEM Owen Avis Pearce and Idris Trevor Richards,

Liverpool,

England, assignors to Associated Telephone & Telegraph Company, Chicago, IlL, a corporation of Delaware Application September 12, 1935, Serial No. 40,200 In Great Britain September 17, 1934 16 Claims.

ators over automatic switches.

The invention is particularly applicable to circuits for the setting up of long distance connections and its primary object is to provide improved arrangements for the automatic selection of outgoing lines and for the giving of special indications to the operators in exchanges handling long distance traffic.

According to one feature of the invention, in a telephone system including a group of lines accessible to two different classes of operators in which in the event of all the lines in a desired group being busy a signal is automatically given to the operators when a line in the group becomes idle, arrangements are provided such \that if operators in both classes are waiting for an idle line the signal indicating that a line in the group is idle is given only to operators in one class.

According toanother feature of the invention, in a telephone system including a group of lines accessible to operators of one class over one group of automatic switches and accessible to operators of another class over another group of automatic switches, in the event of one of said switches finding all the lines in said group busy it'establishes connection with call storing apparatus to which it automatically signals the class of operator making the call.

The invention will be better understood from the following description of one method of carrying it into efiect, reference being had to the accompanying drawings comprising Figs. 1-15. The circuit drawings should be arranged as shown in Fig. 15 to form the complete circuit.

Fig. 1 shows the trunking arrangement;

Figs. 2, 3 and 4 show a jack-ended first trunk selector accessible only to long distance operators;

Figs. 5 and 6 show a second trunk selector.

Figs. '7 and 8 show the call storing equipment accessible to second trunk selectors.

Figs. 9, 10 and 11 show a key-ended first trunk selector accessible only to demand operators and and rotary movement.

A switch of this type is described in the application of Robert N. Saxby, Serial No. 634,265, filed 22nd September 1932.

Referring now to the trunking diagram of Fig.

1 it will be seen that the first trunk selectors TSl accessible to the long distance and demand posi- 5 tions designated LD and DM are provided with two sets of wipers and banks, the lower set being allocated to a large group of trunks outgoing to a distant exchange P. This trunk group is reached by transmitting the digit l which is ab- 7 sorbed in a switching operation to connect up the lower set of wipers after which the switch releases and performs a'level to level hunting operation over the large trunk group. The operator preferably keys up the number into a key sending equipment (not shown) which thereupon sets the automatic switches and during this period the operator is free to attend to other calls.

If all the trunks in the group are busy the wipers rotate on the last level to the 11th step position to connect with the call storing equipmen'CS I. It will be seen from the drawings that the 11th step multiple between the long distance and demand first trunk selectors is disconnected to provide separate routes of access to the call 2 storing equipment in order to provide the necessary discrimination as will be explained in detail later.

Trunks to other exchanges are reached by transmitting a digit other than I as the first digit whereupon a second trunk selector T82 is taken into use which has access to bothway trunk relay sets BW to the desired exchanges. If the second trunk selector encounters a group busy condition the wipers are rotated to the 11th position where they connect with call storing equip ment CS2 similar to that accessible over the first trunk selectors but provided with different supervisory signalling arrangements.

It will first be assumed that a long distance operator wishes to extend a trunk connection to an exchange other than P and for this purpose will insert one of her cord circuit plugs into a jack'extending to a first trunk selector, for instance that shown in Figs. 2, 3 and 4. Relay B connected to the sleeve of the jack is operated over the sleeve circuit of the plug thereby at armatrues b2 and b3 (Fig. 2) lighting the visual engaged lamps VES around the multiple to inform other operators that this particular circuit isin use, at armature b4 connects earth to conductor l0 extending to a traflic recording meter (not shown) and at armature 175 (Fig. 3) operates relay C. Conveniently the lamps VES are lighted from a low voltage alternating current supply of for example 6 volts. Relay C in operating prepares various circuits and in particular completes a hunting circuit to the sender finder switch SF. If for the purpose of the description it is assumed that the wipers of the sender finder are standing in engagement with contacts extending to a sender which is already in use, wiper P will encounter guarding earth on the contact which is extended through the lower winding of relay K to operate the interrupter relay MC. A parallel circuit may also be traced from earth at armature b6 through the upper winding of relay K and as a result relay MC operates but, owing to the high resistance of relay MC relay K remains normal. Relay MC at armature mcl energizes the driving magnet SFM of the sender finder which upon operation releases relay MC at the contacts sfm and interaction takes place between the two to advance the wipers in search of a free sender. When this is found wiper P will encounter a comparatively low resistance battery which shunts relay MC and relay K thereupon operates over both its windings in series to prevent further hunting. Relay K at armature kl opens the circuit to relay MC and completes a holding circuit for itself over its lower winding which is sufficiently low in resistance to mark the selected sender asbusy to other sender finders. Relay K also at armature k extends earth over discriminating wiper D to prevent a register controller finder associated with the sender from hunting for a free register controller since this is not required. Furthermore relay K at other armatures not specifically mentioned extends the incoming circuit from the operator through to the storing equipment of the sender and extends the output circuit from the sender to the impulse responding relay A of the trunk selector. During the rotation of the sender finder, relaylLP (Figure 2) is intermittently operated from the earthed interrupter cam I! and at armature ZpZ short-circuits relay DR thus varying the current in the sleeve circuit and bringing about the flashing of a supervisory lamp in the cord circuit. As soon as relay K switches however, relay A (Fig. 3) is operated in circuit with the sender and in turn operates relay BB which at armature bb3 opens the flashing circuit to relay LP and the lamp in the cord circuit then shows a steady glow to indicate to the operator that a free sender has been found and that keying may commence.

After keying has taken place, the sender commences to transmit over the negative and positive out leads the impulses corresponding to the digits of the number, whereupon relay A (Fig. 3) responds and at armature al repeats the impulses through the low resistance winding of relay C to the vertical magnet VM. Accordingly the vertical magnet raises the wipers to the desired level and on the first vertical step the off-normal springs operate thereby at springs NI completing a locking circuit to relay BB and at springs N4 opening the initial energizing circuit to relay C which remains held over its lower winding in series with the vertical magnet' After the reception of the first digit, relay C releases after its slow period and at armature c3 completes the following self-interrupter driving circuit to the rotary magnet RM: earth at armature m3 (Fig. 4), armatures tl, b8, 03 and h'l, oil-normal springs N5, normal post springs NPI, rotary magnet interrupter springs rm, driving magnet RM to battery. The wipers are therefore rotated over the level selected in search of a free outlet to a second trunk selector and when this is found wiper Pl encounters idle marking battery whereupon relay T operates quickly over both its wind- I ings in series to earth by way of the resistance I2 in parallel with the upper winding of relay H to cut the driving circuit to the rotary magnet at armature ti. At the same time armature tl removes the shunt from the upper winding of relay H which thereupon proceeds to operate in series with the rotary magnet and it will be noticed that the earth connection for this circuit is supplied by way of the resistance l2 in parallel with the lower winding of relay T. Owing to the reversal of the connections the lower winding of relay I now acts in opposition to the upper holding winding but in the circumstances described the holding winding is sufiicient to maintain the relay operated in spite of the. opposition. If, however, two trunk selectors happen to be testing on to the same outlet simultaneously the current in the upper windings of the respective T relays would then be approximately halved and the opposition winding would in such circumstances be effective in bringing about the release of the relay and cause the switch to step on to the next outlet. This arrangement therefore guards against the dual switching of first trunk selectors on to the same second trunk selector and further, due to inherent differences in the operate and release times of the relays it is clear that one of the switches will take possession of the outlet to the exclusion of the other. 7

Returning to the operation of relay H, at armature 7L3 it opens the circuit to relay T and provides a locking circuit for itself over its lower Winding which is sufficiently low in resistance to mark the outlet selected as busy. Further; at armatures hi and 71.2 the negative and positive lines are extended to the second trunk selector and at armature h! the driving circuit is opened to the rotary magnet RM. The sender continues to. transmit the remaining digits to set up the succeeding switches and when it has completed its function it releases and in turn disconnects battery from the P wire to release relay K in the first trunk selector. Relay K thereupon at its resting armatures R2 and k3 extends the calling circuit over the train of switches set up to the called party and also at the resting armature k5 completes a self-interrupter homing circuit tothe sender finder which rotates its wipers to the home contact. a

When the called party replies, battery is connected back over the SI wiper to operate'relay F (Fig. 4) which at armature f2 removes a shortcircuit from the winding of relay DR thereby reducing the current in the sleeve circuit to ex tinguish the supervisory lamp in the cord circuit. Conversely, when the calledparty clears at the end of the conversation, battery is removed from the SI wiper so that relay F releases and increases the currentin the sleeve circuit to light the supervisory lamp. Relay DR it should be explained enables the operator to control the outgoing relay sets which are looked after the receipt of the clear signal from the distant exchange and also to control the release of thefca'll storer relay set. The foregoing operations will be more fully apparent from the subsequent description. I

If, however, all outlets in the level prove busy, the wipers are rotated to the 11th position where the Pl wiper encounters a, battery (not shown) and the selector therefore switches through on to a disconnected outlet. The cam springs are also operated thereby at springs S3 connecting busy tone from lead BT to the upper speaking conductor to inform the operator'that the con-. nection is unavailable.

' In case any abnormal condition should arise during the homing of the sender, earth is connected to the forced release lead FR. which operates relay M. Relay M at armature ml shortcircuits the lower winding of relay K, which thereupon releases, at armature m4 releases relay H and completes a locking circuit for itself, and at armature m5 (Fig. 2) connects number unobtainable tone from conductor NUT to the upper speaking conductor to inform the operator that the connection is unavailable. Under this condition therefore, the first trunk selector only is held and the sender and succeeding switches which may have been taken into use are released.

Upon the release of the connection when the plug is removed from the jack, relay B in the sleeve circuit releases and is followed by relays H and F. A self-interrupter driving circuit is then completed to the rotary magnet from earth connected to conductor [3 via the release signal equipment (not shown) and accordingly the wipers are rotated clear of the banks after which they fall vertically and restore below the banks to the normal condition where the driving circuit is opened at oiT-normal springs N5. The locking circuit of relay BB is opened at off-normal springs NI whereupon relay BB releases to restore the switch circuit to its normal condition. To ensure that the long distance operators receive priority over the demand operators it-is arranged that the first trunk selector extends a discriminating signal to the second trunk selector. When the switch is stepped off-normal relay LP operates over the off-normal springs N6 and locks to its armature ml and later when the switching relay H operates, relay LP is short-circuited by armature hB and releases. During its release period however, a pulse of positive or booster battery is connected by armature Z113 to the SI wiper in order to provide the necessary discrimination as will appear from the subsequent description.

When the operator requires a trunk to exchange P she transmits the discriminating digit l as the first digit so that the wipers are raised to the first level in which position normal post springs NPI (Fig. 3) operate to connect relay DA in place of the rotary magnet so that when relay C releases it causes relay DA to operate. Relay DA looks over its left-hand winding and prepares various circuits in connection with the vertical hunting operation which is to follow and at armature dal completes a driving circuit to the rotary magnet to cause the wipers to restore to normal. When the wipers reach their normal position, relay C is re-operated over the ofi-normal springs N4 and at armature 09 operates relay DB which locks. Relay DB .at armatures db-Z to 11115 connects up the lower set of wipers and at armature db'd completes a self-interrupter driving circuit to the vertical magnet VM as follows: armatures m3, tl, b8, 03, (1176, M2, interrupter springs om, armatures da2, 11b2, cl, lower winding of relay C, winding of vertical magnet VM to battery. Accordingly theswitch performs a vertical hunting operation and when a level containing at least one free trunk to an outgoing relay set is found as marked by battery on the appropriate contact of the vertical'bank AW, relay T operates over armatures c8 and dafi to open the magnet circuit and release relay C. Relay C in turn releases relay T and connects it to the P2 wiper over armatures c8, da5 and dbl, and at the same time a self-interrupter driving circuit is completed to the rotary magnet RM at armature C3. Accordingly the wipers are rotated over the level selected and when a'free outlet is found relays T and H operate in the manner previously explained, this time extending the connection over the lower set of wipers to an outgoing relay set to exchange P. It will'be understood that by the provision of normal post springs arranged to be operated on other levels the switch may be caused to perform a similar hunting operation in response to other digits.

If all the outlets are busy the wipers are rotated over the last level selected to the 11th position where relays T and H switch in the manner previously explained to battery on the P2 conductor leading to call storing equipment for example'that shown in Figs. 12, 13 and 14. At the same time the cam springs are operated thereby at springs S3 (Fig. 2) extending interrupted number unobtainable tone from conductor INUT to the upper speaking conductor to inform the trunk operator of the busy condition, while at springs S2 (Fig. 4) battery is connected to the negative 2 wiper to operate the call storing equipment as will be explained later. The

switch remains in this position and when an out-- let to exchange P becomes free, battery is intermittently connected to the S2 wiper from the call storing equipment which intermittently operates relay F and thus causes the supervisory lamp in the cord circuit to flash. When the operator takes upthe call, the differential relay DR operates due to the change of current in the sleeve circuit and at armature d'rl connects battery to the S2 wiper to unbalance a differential relay in the call storing equipment which accordingly operates. Earth is then connected back over the positive 2 wiper to operate relay RR which looks, and at armature rr2 releases relay H to institute normal release conditions. When the switch is fully restored relay BB is released at the ofi normal springs N4 and relay C re-operates to cause the switch to perform a further hunting operation for the free outlet in a manner similar to that already explained.

Referring now to the call storing equipment shown in Figs. 12, 13 and 14, it will be remembered that this is accessible from both the long distance and demand first trunk selectors, the two routes of access being indicated respectively by upper and lower bracketed sets of conductors extending from Fig.. 12. When all trunks in the group 'are engaged earth is removed from the commonconductor l4 extending to resting armatures of the guard relays in all bothway trunk circuits and relay GP (Fig. 13) which is normally short-circuited thereby then operates and at its armature gpl (Fig. 14) lights the trunk congestion lamps TCL around the operators positions.

If now a long distance operator takes into use a first trunk selector and sets the switch hunting for a free trunk to exchange P the wipers are rotated to the 11th position of the tenth level to connect with the call storing equipment as explained, and relay B (Fig. 12) connected to the P conductor operates in series with the switching relay of the first trunk selector. Relay B prepares various circuits and also at its armatures b2 and M (Fig. 12) disconnects the the trunk free signal thereto while long distance first trunk selectors are waiting. Further, as a result of the connection of battery to the negative conductor from the cam springsof the trunk selector, the differential relay LC (Fig.- 13) is energized over one winding and operates to ad-. Vance the indicating switch INS (Fig. 14) one step in the following manner: relay LC, operates relay R which in turn operates relay V and with both relays R and V operated'the magnet INM of the indicating switch is energized. At the endof the magnet stroke the associated interrupter springs inm release relay LC which in turn releases relays R and V so that the circuit is opened to the magnet INM which de-energizes and advances the wipers to the second position. In this position wiper 1N2 encounters a resistance battery equivalent to that supplied from the first trunk selector and as the differential relay LC is now energized over both its windings with equal current it camiot reoperate. In addition, over wiper 1N4 relay A is operated to light the storage lamp SLl (Fig. 12) and over wiperINS relay Y is operated to disconnect the lighting circuit to the storage lamps SL6-S Ll 0. The storage lamps are situated at the supervisors position andenable her to follow the traflic conditions and-to take whatever steps may be necessary to relieve congestion. The function of relays X and Y associated with the bank'and wiper 1N5 is to economize in the number of relays required for the control of the storage lamps. If a second call is received on the storage group from either a long distanceoperator or a demand operator a further resistance battery is connected to the negative conductor thus doubling the current flowing through the left hand winding of the difierential relay LC which again operates.

Accordingly the indicating switch INS is advanced to the third position in themanner described where wiper IN2 encounters a resistance battery equivalent to that connected to the negative conductor so that thewindings of relay LC again neutralize one another and the relay releases to disconnect the driving circuit; over wiper 1N4 relays A and B are, now operated in series to light the storage lamps SLI and SL2., Up to ten calls may be stored in this manner and it will be appreciated that the resistance connections to the bank 1N2 and bank INS are so arranged to produce one step of the switch INS for each trunk selector connected to the storage equipment. It will also be noticed that relay V is provided with a copper slug to render it slow to release so that it will hold during the stepping of the switch INS to prevent any change in the indicating circuit until the wipers come to rest. It will .be seen from reference to the bank and wiper 1N5 that after six calls have been stored relay Y is released and relay X operated, the former preparing the lighting circuit to the storage lamps SL6 to SLIU while the latter maintains the lighting circuit to the preceding lamps SL2SL5 since all the storage relays except A are now released. Themultiple contacts of the bank INI are led out to terminals i--lil which may becross connected as desired to the common set of terminals connecting with relay OFA. The cross connection is chosen arbitrarily and if it is decided that 10 calls shall be stored before delay working commences the cross connection is made to terminal [0. Accordingly when the wipers of the indicating switch come into this position. and relay V releases, earth from the congestion control key CCK (assumed to be operated to the automatic control position) is extended to relay OFA'which operates andlocks and at armature 0M2 extinguishes the trunk congestion lamps TCL and.

lights the delay lamps DL to indicate to the trunk operators that further calls receivedmust now behandled on a delay. basis. It will beunderstood however, that delay working may be, commenced, at any period at the discretion of the trunk observation operator who can control the lighting circuitof the delay lamps manually by operating the control key CCK to the manual control position shown in the drawings.

When a'trunk becomes free, relay GP is shortcircuited and released by the re-connection of earth to the common conductor l4 and at its armature gp4 connectsrelay LF to earth via the interrupter cam [5. Relay LF therefore intermittently connects battery from the upper winding of relay RR to the S lead to flash the supervisory lamp in the cord circuitin the manner previously explained. The circuit to the lower winding of relay IRR is intermittently interrupted at armature [f2 but as relay RR is differentially wound it does not operate while both its windings are energized with currents of equal strength. The circuit to the congestion lamps is maintained by relay OFA. When the free trunk is taken up by an operator, battery is connected to the S lead in the trunk selector thereby reducing the current in the upper winding of relay IRR which thereupon becomes unbalanced and operates, and at armature irr2 opens the circuit to relay LF and at armature irrl connects earth to. the positive conductor to release the trunk selector in question, after which it performs a further hunting operation in the manner previously described to select the free trunk. Re-

sistance battery on the negative conductor corresponding to this switch is now removed so that the balance of relay LC is again upset and it releases thus causing the indicating switch INS to perform a further hunting operation to a position corresponding to the number of calls still waiting. Relay GP re-operates when earth is removed from conductor M to maintain the circuit to the trunk congestion lamps, at armature and opens a further point in the circuit to relay LF and at armatures gp2 and 9113 releases relay IRR.

If it is assumed that both long distance trunk selectors and demand trunk selectors are in engagement with the call storing equipment, it will be seen that so long as relay B remains operated the S lead to the demand trunks selectors is opened and therefore the trunk free flash signal is extended only to long distance operators. When all the long distance trunk selectors have been re-set relay B releases and the flash signal is then extended to the demand operators.

Referring now to the second trunk selector shown in Figs. 5 and 6, when the switch is taken into use relayA operates over the negative and positive conductors and relay B operates over the P wire in series withthe switching relay of thefirst trunk selector. At armature b5 relay B closesa circuit for a .trafiiorecorder (not shown). Relay A operates relay BB, and relay B at armature b4 operates relay C, to prepare the circuit for vertical impulsing. In response to the second impulse train, relay A is intermittently operated and at its armature al repeats the impulses through the low resistance upper winding of relay C tothe vertical magnet which accordingly raises the wipers to the desired level. On the first vertical step the cit-normal springs operate thereby at springs N2 opening the initial energizing circuit to the lower winding of relay C which however, remains held during impulsing by reason of the copper slug on its core. Relay BB also holds during impulsing. At the end of the impulse train, relay A holds operated and relay C releases after a short period to operate relay E over armature C2 and the off-normal springs N2. Relay E at armature e4 transfers the impulsing circuit to the rotary magnet RM and at armature 6'! reoperates relay C which at armature 02 transfers the energizing circuit for relay E to the control of its own armature el.

If hunting. is to take place over the selected level, the digit 1 is then received and repeated through the low resistance Winding of relay C to the rotary magnet which advances the wipers into engagement with the first contact of the level selected. Rotary off-normal springs NR2 operate during the first rotary step to open the energizing circuit of relay C so that this relay releases a short period after the end of the impulse and in turn releases .relayE. During the slow release period of relay E, relay PS is operated over its. lower'winding, bank and. wiper Rand armatures b't3,.c4 and e5 to resistance battery and thereupon looks over its upper Winding for the remainder of the connection. Relay PS prepares various circuits which will not be specifically mentioned at this time and completes the following self-interrupter driving circuit to the rotary magnet RM which becomes efiective upon the release of relay E: earth at armature 71'! (Fig. 6), armature ti, ps2, bl, 1'13, 05, ps8, 66, springs NI,

* interrupter springs rm, winding of rotary magnet to battery. Rotary hunting therefore commences and whena free outlet is found as indicated by marking battery on the P contact, relay T is operated quickly over both its windings in series to earth via the'low resistance It and at armature tl opens the driving circuit to the rotary magnet to. prevent further movement of the wipers. Atthe same time armature it removes a short-circuitfrom relay H which proceeds to operate over its upper winding in series with rotary magnet to earth derived from the resistance I6 connected in parallel with the left-hand winding .of relay T. The current through the lefthand winding of. relay T is reversed under these conditions. and the relay performsa dual testing function in the manner described in connection With the first trunk selector. Relay H at armaturehfi disconnects relay T and provides a locking circuit for itself over its lower winding which is sufficiently: low in resistance to mark the selected outlet as busy. Furthermore, at armatures hl, 71.2 and 115 the negative, positive and S leadsrespectively; are switched through to the trunk line circuit and the calling operator requests thedesir'ed connection from this distant operator.

If all outlets in the level selected are engaged, the wipers rotate to the 11th position where the cam springs operate and at cam springs S3 a resistancebattery is connected up to bring about the operation'of relay H which switches the calling circuit through to the call storing equipment shown in Figs. '7 and 8. Moreover, at cam jsprings'S2 battery is extended over the S wiper lturebtiin conjunction with the cam springs S5 hunting connects relay RS in bridge between the positive wiper and the P wiper.

When a trunk becomes free, alternating current is fed from the call storing equipment over a circuit connected between the positive and P leads and operates relay RS which it will be noted is connected in parallel with a metal rectifier MRB to enable it to hold operated continuously from alternating current. Relay RS at armatures T51 and 1s2 completes circuits for the upper and middle windings of relay RR and at armature 7'83 connects relay LF (Fig. 5) to earth by Way of the interrupter cam. Relay LF therefore operates intermittently and interrupts the battery connection from relay RR over the S conductor to the first trunk selector where it is repeated to bring about the flashing of the supervisory lamp in the operators circuit. At the same time the connection to the centre winding of relay RR is also intermittently interrupted and as the relay is differential it does not operate under these conditions. When one of the waiting operators takes up the trunk a further battery connection is made to the S conductor in the first trunk selector, this reduces the current in the upper winding .of relay RR whereupon this relay operates, at

armature 71'4 it releases relay LF and at armature TTZ holds over its lower winding in series with the vertical magnet VM which is also operated. The

operation of the vertical magnet unlatches the rotary detent in the manner described in the previously-mentioned Saxby application to allow the wipers to release in a rotary direction only. When the Wipers leave the 11th position relay RS releases and when they reach their rotary normal position the normal rotary springs NRI are again restored and relay RR is released. The rotary is therefore re-commenced and the switch now searches for a free line in the manner already described.

If a particular line in the group is to be selected for example for testing purposes, the corresponding digit is dialled in place of the digit 1 and the wipers move to the required line in the selected level. If the line is free relay H operates during the slow release period of relay E in the following circuit: earth, resistance I6, upper winding of relay H, armatures bl, TT3, 05, p.98 and 62 to resistance battery. Relay H locks over its lower winding and armatures I116, c5, c4 and 11153 to the P wiper to busy the line and complete the connection through to the trunk line circuit. If the line is busy relay H is not operated and upon the release of relay E relay BT operates over armatures ps3, e3, M2, 714 and 124 to connect busy tone to the upper speaking conductor from conductor BT and intermittent battery to the S conductor extending to the first trunk selector.

In case the switch has been taken into use by a long distance operator'the booster pulse on the S conductor which is extended during the switching of the first trunk selector operates relay PF via the metal rectifier MRA and this relay thereupon locks over its right-hand winding. If the call goes on to the call storing equipment the fact that relay PF is operated causes earth to be extended over the negative Wiper to operate a discriminating relay in the call storing equipment and at the same time the connections to relay RS are re-arranged.

When a trunk line becomes free the call storing equipment 'now connects earth to the P wiper alone so that the RS relays are operated in a direct current circuit but only those second trunk selectors which have been taken in use from a long distance position. Consequently only long distance positions receive the trunk free signal and are thus given preference over the demand positions.

Normal post springs NP are provided-so that they may be operated on certain levels to cause the switch to rotate. automatically after the receipt of the first train of impulses if this condition is required. When these springs are closed relay PS is operated immediately upon the release of relay C and at once institutes the rotary hunting condition.

Figs. 7 and 8 show the call storing equipment accessible to second trunk selectors which is similar in operation to the call storing equipment shown in Figs. 12, 13 and 14. Although it has only one route of access it is provided with a discriminating relay PF which operates when a call is extended by a long distance operator to alter the supervisory signalling circuit so that when a trunk becomes. free only the long distance operators are signalled as previously explained. When relay PF is normal, as in the case of demand working, the supervisory signal comprises induced alternating current applied between the positive and P conductors. When relay PF is operated as in cases. where the call storing equipment has been taken into use by a long distance operator the supervisory signal comprises direct earth connection to the P conductor alone.

Figs. 9, 10 and 11 show the first trunk selector accessible from demand positions, this is substantially identical in operation to the first trunk selector of the long distance positions shown in Figs. 2, 3 and 4 except that it is arranged to be taken into use by the operator throwing a key instead of plugging into a jack. It will also be noticed that relay LP has been omitted and there is therefore, no provision for sending a booster signal to the S conductor as in the case of the long distance trunk selector.

Throughout the foregoing circuits the resistance-condenser connections made to relay coils, magnets and across interrupter springs are provided for spark quench purposes.

We claim:

1. In a telephone system, a group of lines, means for giving operators of two classes access to said lines, means for storing incompleted connections resulting from attempted calls over said lines at a time when the same are all busy, and means responsive to a line becoming idle for transmitting an idle. line signal to waiting operators of one class only.

2. In a telephone system, a group of lines accessible to two different classes of operators, the means of access including automatic switches adapted to hunt over said group of lines, call storing apparatus with which said switches automatically establish connection responsive to calls for lines in said group when the same are all busy, and means included in said apparatus responsive to a line of said group becoming idle for signalling the fact to the operators of one class to the exclusion of the operators of the other class.

3. In a telephone system, a trunk leading from an operators switchboard to a selector, a group of trunks accessible to said selector, means for operating said selector responsive to a digit transmitted over said first trunk to cause the selector to hunt for an idle trunk in said group, means for holding the incomplete connection if all trunks in said group are found busy, means for transmitting a free signal over said first trunk to the operator when a trunk in said group becomes idle, and means responsive to a control condition imposed on said first trunk by the operator for releasing said selector and for automatically causing the same. to rehunt over said group to find the idle trunk.

4. In a telephone system, two groups of first selectors for use by two classes of operators, respectively, a group of second selectors accessible to both said groups of first selectors, means in each second selector responsive to selection thereof by a first selector in the group used by a particular class of operators for registering such selection, call storing apparatus accessible to said second selectors if a selected group of trunks is busy, and means in each second selector. controlled by said registering means for transmitting to said call storing apparatus a signal indicative of the class of operator engaged in the call.

5. In a telephone system, two groups of first selectors for use by two classes of operators, respectively, a group of second selectors accessible to both said groups of first selectors, means in each second selector responsive to selection thereof by a first selector in the group used by a particular class of operators for registering such selection, call storing apparatus accessible to said second selectors if a selected group of trunks is busy, means in each second selector controlled by said registering means for transmitting to said call storing apparatus a signal indicative of the class of operator engaged in the call, means in the call storing apparatus responsive to an ensuing idle trunk condition for sending a free signal of predetermined character to the second selectors connected therewith, and free signal receiving means ineach second selector prearranged to respond or not dependent on whether the associated registering means has been operated.

6. In a telephone system, first se1ectors,'second selectors, a group of trunks accessible to an operator through said first selectors, a second group of trunks accessible to said operator through said first selectors and second selectors, a call storing apparatus accessible to said first selectors if all trunks of said first group are busy, means in said apparatus for sending a signal to a connected first selector responsive to a trunk in said first group becoming idle, a call storing apparatus accessible to said second selectors if all trunks in said second group are busy, means for sending a signal to a connected second 'selector from said second apparatus responsive to a trunk in said second group becoming idle, and means in the connected second selector responsive to the signal received from said second storing apparatus for transmitting to the connected first selector a signal identical with the signal which the first selectors receive from said first storing apparatus. I

7. In a telephone system, selectors for use in common by two classes of operators, a group of trunks accessible to said selectors, call storing apparatus accessible to said selectors if operated at a time when said trunks are all busy, means in each selector effective when in use by a. particular class of operator'for conditioning itself to receive a particular kind of signal and for indicating to the storing apparatus the kind'of signal it is adapted to receive, and means in said storing apparatus responsive to a trunk in said group becoming idle for transmitting to all selectors connected therewith a signal of the char- 

